Hmmm, what about...
Upstream Color (2013) by Shane Carruth -- If anyone else has actually seen this, I bet you can't come up with a coherent plot description. It's something about music and something about DNA and something about the cycle of life and something about pigs. And you thought Primer was confusing! I probably need to watch this again.
You Were Never Really Here (2018) by Lynne Ramsay -- I just saw this one in the theater recently. Some people are going to hate it but I actually thought it did a masterful job of making you empathize with a character that most people wouldn't find very empathetic. I don't want to spoil anything, but the first 20 minutes of this movie in particular are a total mindtrip.
I'm tempted to put something by Gasper Noe here since mindtrip movies are his Raison d'etre but I don't actually recommend any of them. So how about a different French movie instead which is actually watchable...
Last Year at Marienbad (1961) by Alain Resnais -- A couple stroll in a garden. He claims they met in the same location a year ago but she has no memory of this. Is he lying? It's either a film about a character trying to manipulate another character or a film about a filmmaker trying to manipulate the audience or a film about the unreliability of memory. Or maybe it's just a series of images strung together in a manner that simulates meaning without actually meaning anything? Heck if I know.